As the compounds F124 and F125 can be used as substitutes for perchlorofluorocarbons (CFCs) in the field of aerosols (propellants) and in that of refrigeration, there is currently a search for highly efficient processes for their industrial production.
U.S. Pat. No. 4,766,260 describes a process for synthesis of compounds F123 and F124 by hydrofluorination of perhalogenated olefins in the gas phase, the object being to minimize the formation of F125. Example 13 (column 6) describes the fluorination of tetrachloroethylene with a catalyst CrCl.sub.3 /Al.sub.2l O.sub.3 ; despite a temperature of 350.degree. C., a long contact time (60 seconds) and a high HF/C.sub.2 Cl.sub.4 molar ratio (6/1), the selectivities towards F124 and F125 are low (33.3% and 7.2% respectively).
The use of a catalyst based on chromium(III) (CrCl.sub.3) supported on carbon for the gas phase catalytic fluorination of halogenated olefins forms the subject of the Japanese Patent Application published under No. 48-72105/73 in which Example 4 describes the fluorination of tetrachloroethylene. There again, despite a reaction temperature of 400.degree. C. and a high (5/1) HF/C.sub.2 Cl.sub.4 molar ratio, the composition of the products formed is limited to F121 (CHCl.sub.2 -CFCl.sub.2 : 6.8%), to F122 (CHCl.sub.2 -CClF.sub.2 : 10.5%) and to F123 (82.7%).
U.S. Pat. No. 3,258,500 describes the use of mass chromium or chromium supported on alumina for gas phase catalytic fluorination reactions. In particular, Example 17 (column 14) describes the fluorination of tetrachloroethylene. At 400.degree. C., with a HF/C.sub.2 Cl.sub.4 molar ratio of 6.2/1, the selectivity towards F123+F124+F125 is low (47.7%); a reduction in the reaction temperature (300.degree. C.) improves this selectivity (79.7%) but the distribution is then shifted towards the less fluorinated products (F123 and F124).
Patent Application EP 0,349,298 describes the synthesis of the compounds F123 and F124 from pentahaloethanes by gas phase catalytic fluorination over a catalyst composed of a metal chosen from chromium, cobalt, nickel and manganese deposited on alumina. This document puts the emphasis, on the one hand, on the exhaustive activation of the catalyst with hydrofluoric acid (at least 90% of the support in the form of AlF.sub.3 after activation) and, on the other hand, the minimized formation of F125 during the reaction. Thus, in Example 6 which describes the fluorination of F122 at 350.degree. C. and with a long contact time (30 seconds), the selectivity towards F125 is only 1.1% and the cumulative selectivity (F123+F124+F125) is only 71.5%. In Example 5 which describes the fluorination of F123 in the gas phase over a NiCl.sub.2 /Al.sub.2 O.sub.3 catalyst at 400.degree. C., with a contact time of 30 seconds and a HF/F123 molar ratio of 4, the selectivity towards F125 is only 7.5%.
A process for producing fluorinated aliphatic hydrocarbons, based on the fluorination in the gas phase by hydrofluoric acid of halogenated aliphatic hydrocarbons containing at least one halogen atom other than fluorine, forms the subject of U.S. Pat. No. 3,755,477 where the catalyst is a mass chromium oxide treated with steam before calcination and activation with hydrofluoric acid. Example 25 uses such a catalyst for the fluorination of F123 at 390.degree. C. with a high (9.5/1) HF/F123 molar ratio; the selectivities towards F125 and F124 are respectively 67 and 21% but a selectivity of 2.5% towards chloropentafluoroethane (F115), which can not be recycled, is also observed.
From the examination of the state of the art, it appears difficult to synthesize the two desired compounds (F124 and F125) with good selectivity and significant productivity by direct fluorination of tetrachloroethylene or of F122. Starting from tetrachloroethylene, and despite high contact times, temperatures and molar ratios, it seems difficult to obtain F124 and more especially F125 with good yields. The synthesis of these two compounds is easier from F122 but, in this case, a problem of selectivity is encountered (significant formation of by-products).
As regards U.S. Pat. No. 3,755,477, it shows that, from F123, the synthesis of F125 requires a high molar ratio (9.5) and a high temperature (390.degree. C.) leading to the not insignificant concomitant formation of undesirable F115.
Patent Application WO 92/16482 relates to the manufacture of F124 and of F125 by gas phase fluorination of pentahaloethanes, especially that of F123, over a catalyst based on zinc and optionally on another metal deposited on a fluorinated alumina. Despite a high contact time (30 seconds), the best degree of conversion of the F123 obtained in the examples is approximately 60% and the maximum selectivity towards F125 is 28%.
Patent Application FR 2,669,022 describes the use of a catalyst based on nickel and chromium supported on AlF.sub.3 or fluorinated alumina for the specific fluorination of F133a (CF.sub.3 -CH.sub.2 Cl) to F134a (CF.sub.3 -CH.sub.2 F), this catalyst making it possible to obtain very good selectivities towards F134a.
In view of the advantage of the compounds F124 and F125 as substitutes for CFCs, their industrial manufacture requires a particularly highly efficient process, that is to say one which makes it possible to obtain:
a very high selectivity towards F124+F125 PA0 a high productivity of F124 and/or F125 PA0 a high flexibility to direct, as desired, the manufacture towards the production, as the major product, of F124 or towards that of F125, while minimizing the formation of by-products. PA0 drying at low temperature (100.degree. to 150.degree. C., preferably 110.degree. to 130.degree. C.) in the presence of air or nitrogen, PA0 drying at high temperature (250.degree. to 450.degree. C., preferably 300.degree. to 350.degree. C.) under nitrogen or under air, PA0 fluorination at low temperature (180.degree. to 300.degree. C., preferably at approximately 200.degree. C.) by means of a mixture of hydrofluoric acid and nitrogen, the HF content being controlled so that the temperature does not exceed 300.degree. C., and PA0 finishing under a stream of pure hydrofluoric acid or hydrofluoric acid diluted with nitrogen at a temperature which can range up to 450.degree. C.
A process which makes it possible to achieve this object was described in Patent Application FR 2,661,906 which, for the fluorination of F123, recommends a chromium oxide catalyst supported on active charcoal.